Such liquids may be for example fragrance compositions such as described in US 2005/0037945, or insecticides or still other liquids having a relatively high viscosity.
Such liquid droplet spray devices are also sometimes called piezoelectric spray devices, aerosol generators, nebulizers and the like. They normally contain a nozzle body on a support part, in particular, a nozzle body of a liquid droplet spray device which dispenses a liquid substance as a liquid droplet spray. They further consist of a piezoelectric actuator used as vibrating means for causing the liquid to vibrate so as to be accelerated and expelled as droplets. They further consist of elements such as a liquid space, liquid feed and fluid interface linked to a reservoir, a reservoir as well as electrical connections between the ultrasound generating means and a corresponding electronic circuitry. The liquid may be for example an ambient fragrance, a perfume, an insecticide, an aromatherapy essence, a liquid pharmaceutical formulation, aqueous based liquids and flammable or combustible liquids.
Such nozzle bodies are sometimes called aperture plates, nozzle arrays, dosing aperture, orifice plate, vibratable membrane member, dosing aperture arrangement, aerosol generator and the like. Such terms are hence to be understood as being interchangeable throughout the present document.
In fact such nozzle bodies and droplet spray devices are well known. For example see the document EP 1 129 741 in the name of the present Applicant. This document describes a liquid droplet spray device having a top substrate formed of a main body and of a nozzle body. The nozzle body contains a nozzle array of liquid droplet outlet means allowing a liquid substance contained in the liquid droplet spray device to exit the device, in this case as a spray of droplets. A piezoelectric actuator is used to cause the liquid to undergo a vibration so as to generate the droplet spray.
It is well known to disperse fragrances and insecticides into the atmosphere by such device to form a mist or cloud of small droplets of a liquid substance containing the fragrance or insecticide and to eject the mist or cloud into the atmosphere in the form of minute liquid droplets. As the mist or cloud settles, the fragrance or insecticide evaporates from the droplets. Examples of devices for doing this are shown in U.S. Pat. No. 4,085,893, U.S. Pat. No. 5,173,274, U.S. Pat. No. 5,601,235 and U.S. Pat. No. 5,894,001. In general these devices supply the liquid fragrance or insecticide to a vibrating atomization plate which, due to its vibrations, breaks up the liquid into fine droplets and ejects them upwardly in the form of a mist or a cloud. As the droplets fall back down, the fragrance or insecticide evaporates from the droplets and disperses into the atmosphere.
Document WO 2008/015394 describes a spray device without an atomiser plate, but instead uses a wick fixedly attached to a vibrating element. When the vibrating element is activated, the wick will vibrate together with the vibrating element so that any liquid absorbed by the wick will be dispersed therefrom. Such device may avoid clogging, due to the absence of an atomiser plate, but the disclosure is totally silent about fallback.
Document WO 00/47335 describes another example of such a device, and a corresponding method for dispensing liquids of relatively low viscosity. The described device uses a dome-shaped vibrating orifice plate that is actuated by a piezoelectric vibrating element to vibrate the plate. Once the plate vibrates, liquid is supplied to the plate by capillary action so as to be expelled there from as a spray of droplets. As described in this document, liquids having a viscosity of less than 5 centipoises may be used, but at higher viscosity, the device stops functioning.
A problem occurs in the operation of these known devices in that there is no means to be sure that all of the liquid which is ejected will indeed evaporate before the droplets fall back onto surrounding surfaces. As a result, an often destructive liquid residue of unevaporated liquid builds up on these surfaces. This problem is particularly difficult where the liquid to be ejected is a fragrance or an insecticide having a relatively high viscosity. This is because fragrance and insecticide compositions are generally quite complex; and there has been no way to know in advance that a particular composition will fully evaporate when subjected to atomization in a vibrating plate atomizer.
Document WO 2007/054920 describes a further example of such device having a vibratable atomizer plate. Here, an air disturbance generator, such as a fan, is proposed to increase the ability of the ejected mist of droplets to remain airborne so as to increase the time allowing for increased vaporization of the droplet mist. The ejected droplets are compositions having lower and higher volatile components, and only the highly volatile components remain airborne, but the other components fall back down onto the surrounding surface. Further, a fan constitutes an additional component that needs to be placed correctly and adds to the constructional costs of the device.
A solution has been proposed in view of these problems by analysing the vapour pressure of the components of the sprayed liquid. Indeed, according to U.S. Pat. No. 6,793,149 and U.S. Pat. No. 7,070,121, when liquid compositions are broken up into small droplets and ejected into the atmosphere above a surrounding surface, such as a tabletop, for example, the ability of those droplets to become fully evaporated before they fall back onto the surrounding surface, does not depend on the vapour pressure of the liquid composition itself. Instead the ability of the droplets to evaporate depends upon the vapour pressures of the individual components of the liquid composition. This document thus describes a method of nebulising a multi-component liquid solution having a high-viscosity, such as a multi-component liquid insecticide or a perfume in a manner such that the amount of liquid deposited on adjacent surfaces is minimized. The liquid solution comprises a plurality of components having respective vapour pressures. The described method is based on the discovery that the vapour pressure of the lowest vapour pressure component of the liquid composition must be such that this component will evaporate before the liquid droplet containing this component reaches the surrounding surface.
However, according to this same document, the composition of liquids is not always exactly known. Where liquid compositions, such as fragrances or insecticides, comprise large numbers of components, it is often not practical to ascertain the vapour pressures of each of the individual components in order to determine the evaporation characteristics of nebulised droplets of the liquid. Further, often, the liquid components themselves and their respective concentrations are not known because the fragrance or insecticide is maintained by the supplier as a trade secret.
In such cases, it is thus impossible to use the described method.
It is, therefore, an object of the present invention to provide an innovative method and liquid droplet spray device for controlling the evaporation of the droplet cloud resulting from nebulising a liquid that overcome the inconveniences and limitations presented by the prior art documents, and that allow to minimise the fall back for liquids such as fragrances or the like.